A new species of Compsoneuriella Ulmer, 1939 (Ephemeroptera: Heptageniidae) from Thailand.

A new species of Compsoneuriella Ulmer, 1939 is described based on male and female imagos, nymphs and eggs from Ratchaburi province, western Thailand. Compsoneuriella braaschi sp. nov. is closely related to C. thienemanni Ulmer, 1939, from Java and Sumatra, but differs in the larval stage by the maxilla bearing only 8-9 comb-shaped setae on the crown, two strong medial setae on the glossa, gills V-VII apically acutely pointed and the posterior margin of the abdominal terga without distinct rows of submarginal microdenticles. In the male imaginal stage, the new species differs from C. thienemanni by having more reduced spines on the penis lobes and the titillators being directed outwards and strongly pointed at their apices.